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Date: Fri, 22 Mar 2024 09:28:42 -0300
From: Jason Gunthorpe <jgg@nvidia.com>
To: peterx@redhat.com
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	linuxppc-dev@lists.ozlabs.org,
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Subject: Re: [PATCH v3 05/12] mm/gup: Drop folio_fast_pin_allowed() in hugepd
 processing
Message-ID: <20240322122842.GH159172@nvidia.com>
References: <20240321220802.679544-1-peterx@redhat.com>
 <20240321220802.679544-6-peterx@redhat.com>
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On Thu, Mar 21, 2024 at 06:07:55PM -0400, peterx@redhat.com wrote:
> From: Peter Xu <peterx@redhat.com>
> 
> Hugepd format for GUP is only used in PowerPC with hugetlbfs.  There are
> some kernel usage of hugepd (can refer to hugepd_populate_kernel() for
> PPC_8XX), however those pages are not candidates for GUP.
> 
> Commit a6e79df92e4a ("mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
> file-backed mappings") added a check to fail gup-fast if there's potential
> risk of violating GUP over writeback file systems.  That should never apply
> to hugepd.  Considering that hugepd is an old format (and even
> software-only), there's no plan to extend hugepd into other file typed
> memories that is prone to the same issue.
> 
> Drop that check, not only because it'll never be true for hugepd per any
> known plan, but also it paves way for reusing the function outside
> fast-gup.
> 
> To make sure we'll still remember this issue just in case hugepd will be
> extended to support non-hugetlbfs memories, add a rich comment above
> gup_huge_pd(), explaining the issue with proper references.
> 
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: Lorenzo Stoakes <lstoakes@gmail.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: linuxppc-dev@lists.ozlabs.org
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  mm/gup.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Jason